As defined by Nugent’s Gram stain score (20
), our study confirms that women may have short episodes of disruption in vaginal microbiota that spontaneously resolve. It was surprising that half of enrolled women had rapid fluctuation in their vaginal microbial communities. This observation reveals a weakness of traditional cross-sectional or long-interval sampling studies, as BV episodes will be missed if samples are collected on a single observation or are collected weekly, monthly, or less frequently. Such studies would underestimate the incidence of BV and the number of recurrences. Our data also indicate that physical disturbances may be associated with BV onset. It is, however, not clear why disruption of the microbiota is more prevalent in some women after such activities (or menses) but not in all women. We hypothesize that vaginal microbial community types vary between women and that a number of community types are more susceptible to disturbance and subsequent risk for BV.
Poor sampling technique or observation error cannot account entirely for our observed fluctuations in Nugent scores. Several longitudinal studies with daily sampling have documented rapid fluctuation in Gram stain vaginal smears. (13
Lubricant use was a strong predictor of BV onset in this study. Lubricants contain a wide variety of ingredients, including glycerin and chlorhexidine. Chlorhexidine is a broad spectrum microbicide that may cause significant toxic effects to mucosal surfaces. Glycerin may increase local osmolarity and its effect on protective lactic acid producing bacteria is not known.
We also found that rectal sex was associated with BV onset. This association was inferred from only 5 women, but the point estimate was significant with a reasonably tight 95% confidence interval. Transfer of microorganisms from the rectum to the vagina may disrupt the vaginal equilibrium or induce local inflammatory responses that may lead to an increased susceptibility to vaginal microbiota disruption and BV. Despite the frequent report of this sexual practice by women (25% of participants in the Parent study), there are few and conflicting data on the association between rectal sex and BV. (26
In the current analysis, douching the day prior was marginally associated with BV onset. Because samples were collected every 3 days, the study design was not be sufficient to determine if the observed association was causal or due to women douching in response to BV symptoms.
A strength and unique aspect of our study is the analytical method which allows each woman to serve as her own control. Therefore, we do not need to control for time-independent factors which vary between women (such as age or medical history). However, a limitation of this case-crossover approach is that only women who have discordant pairs (women with fluctuating BV status) contribute to the analytic model. Our results, however, are consistent with standard between-woman evaluations of time-varying risk factors. (12
There are several limitations to our study. Time-varying exposures noted as associated with BV onset are hypothesis-generating and therefore were not corrected for multiple comparisons. Our findings were data driven. It is not known what time frame, whether one or three days prior to sampling, should be used to evaluate different intravaginal exposures. In addition, daily information on vaginal symptoms was not collected. As the Parent study was a longitudinal field-based study with self-sampling, the study design did not include evaluation for BV by Amsel’s clinical criteria. (22
) The study population may also not be generalizable. The study recruited women who reported use of vaginal douche products, and this population may represent a group of women who have increased variability in their vaginal microbiota. However, the reasons for douching were varied (19
) and as demonstrated by 27% of women persisting with normal Nugent scores throughout the 16 weeks of observation, women with both disturbed and undisturbed vaginal microbiota were sampled. African American women were over-represented in the cohort and they are at higher risk for BV (6
) and potentially recurrent BV. Fifty-seven percent of this cohort had at least one episode of BV, while data from a nationally representative- survey found a prevalence of BV of approximately 38% among U.S. women who reported recent douching. (6
) Finally, risk for BV may be modulated by contraceptive method (15
) and 45% of participants reported a history of tubal ligation.
Both the etiology of BV and the vaginal microbiome remain poorly understood. It is not known how the transient fluctuations observed on Gram stain reflect a clinician’s diagnosis of BV. The fluctuations may represent temporary states of limited clinical significance. However, future research on the implications of longitudinal changes in vaginal microbiota is warranted. If a healthy microbiota is a protective state and a disrupted microbiota is viewed as a non-protective state (guards down) in part because of the bactericidal and virucidal actions of lactic acid, fluctuations of vaginal microbial communities may result in intervals of increased susceptibility to STIs and HIV. (3
) Use of new molecular technologies will facilitate a more comprehensive understanding of the changes in vaginal microbial populations that characterize BV and the differences in microbial species composition and abundance that occur between healthy and BV-prone microbiota.
Together with prior studies (13
), we have highlighted the dynamic nature of the vaginal microbiota and demonstrated the need for frequent prospective sampling in order to significantly advance our understanding of the dynamics of vaginal microbiota structure and function, and the microbiological, biochemical, molecular, and behavioral contributions to onset and remission of BV.